Los Angeles is a seasonable southern city on the west coast. Its average monthly maximum temperatures are historically confined to the 64-77 ºF range year round [source]. Unsurprisingly, the big concern for residents in terms of energy efficiency is the cost of air conditioning. One of the major sources of heat entering the house is through window light. So southern Californians, like residents of other longitudinally southern U.S. states, have taken to applying reflective windows treatments, both as a cost-saving measure and as a "green" energy saver.

But in this case the approach backfired, as the neighbors' energy efficient reflective gloss created a brilliant beam during daytime hours that directed itself directly at the car port.

"The side view mirrors were melting. Anything that was plastic on the car was melting," the Prius-owner recalls.

And Ms. Patron says her Prius wasn't the only ones damaged -- a neighboring vehicle saw similar melting. She comments, "I'm positive that this window is what is causing the damage to my car. I just don't feel like it’s fair. I feel like it needs to be known that this is happening. And a lot of people probably have damage out there, that they aren't aware that it’s the windows that are causing this."

She claims to have measured the temperature in the car port when it's exposed to the beam and found it to be 120 ºF.

Similar claims of reflective windows burning people or plastics were leveled in Sept. 2010 by a lawyer staying at the Vdara hotel in Las Vegas, Nevada. Reportedly in that case the situation was even more severe as the reflective windows were acting as a hyperbolic solar dish, focusing sunlight into a "death ray", which swept across the pool deck.

Reportedly other homeowners have also complained about plastic-melting reflective beams coming off windows with certain energy-efficient treatments. The National Association of Home Builders has promised a thorough investigation into this problem.

What makes Ms. Patron's story more curious is that automotive mirror housings are usually primarily composed of ABS (acrylonitrile butadiene styrene) [source] -- the same type of plastic found in LEGO blocks and plastic pipes. Pure ABS has a melting point of 221 ºF [source], so it's unclear how the beam was able to melt the plastic, even if the air temperature was 120 ºF.

One possibility was that the black mirror plastic absorbed the light, and heated up to a temperature far hotter than the surrounding air (think of an Easy-Bake oven).

The Prius is the best-selling hybrid electric vehicle in the U.S. (and in the world).
[Image Source: Jason Mick/DailyTech]

the windows have absolutely NOTHING to do with the melting mirrors and in fact this was caused entirely by an act of vandalism? The reflective coatings on windows do not shine the light in any particular direction but instead diffuse the light in all directions so I fail to see how the window can act as a parabolic mirror to which it actually melts plastic. Even if this window was actually a mirror and it was angled just so with the sun, because it's a flat window, it just isn't going to reflect enough light to cause melting of the side mirror.

One thing to note, panes of glass used in windows are rarely completely optically flat, plus they flex when installed so they can curve into a concave shape easily. Also you don't need but a very small amount of curvature to focus at long distances.